Project Summary/Abstract Rumination, a perseverative cognitive process that involves dwelling on negative emotions, is a transdiagnositic risk factor for the development of several common forms psychopathology including depression, anxiety, and self-injurious behaviors (Nolen-Hoeksem, Wisco, & Lyubomirsky, 2008). Thus, reducing the tendency to ruminate in adolescence, a time when many psychological disorders often first develop, has the potential for a large public health impact. Mindfulness meditation involves focusing on the present moment without judgment, and interventions using this strategy have been associated with reductions in trait rumination (Chiesa & Serretti, 2009). Most mindfulness interventions involve several weeks of training; however, a brief mindfulness intervention was shown to reduce state rumination in adolescents (Hilt & Pollak, 2012), suggesting that a more accessible, less intensive intervention format may be effective. Because adolescents often consume media on mobile devices, the proposed study tests a newly-developed mindfulness mobile application designed to reduce rumination. One hundred fifty high-ruminating adolescents will be randomly assigned to the 3-week intervention that involves ecological momentary assessment (EMA) and mindfulness strategies or a control group who only complete the EMA. Data on trait rumination and symptoms of psychopathology among adolescents will be collected from all participants and their parents at baseline, post- intervention, and at two follow-up periods. Aim 1 will determine whether the intervention effectively engages youth by assessing its acceptability. Acceptability will be determined through questionnaires along with assessment of compliance and continued usage beyond the intervention period through electronic monitoring during a 3-month follow up period. Aim 2 will examine the efficacy of the intervention, with the mindfulness group expected to have reduced symptoms at post and follow-up periods relative to the EMA-only group, with reduced state and/or trait rumination expected to mediate the reduction in symptoms. This preventive intervention has the potential for a large public health impact by delivering a cost-effective intervention that targets a transdiagnostic risk factor during adolescence. The project will also serve as a proof of concept for mobile mindfulness applications and mobile mental health applications more broadly, which currently lack rigorous empirical testing.